Delphi DLL

Large Persistent Hash Table Stored on Filesystem

See more Misc Examples

Demonstrates how to implement a large, persistent hash table that is stored on the filesystem and allows for quick retrieval using a hash key. Delphi DLL

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Cache;

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Boolean;
cache: HCkCache;
key: PWideChar;
eTag: PWideChar;
itemValue: PWideChar;

begin
success := False;

// Let's say we would like to implement a persistent hash table with approximately 200,000 entries.
// We want something simple and straightforward.
// 
// The Chilkat Cache class is a solution that might fit.
// 
// Each hash table entry is a file.  Depending on the number of anticipated hash entries,
// the files can be contained in a single directory, or a collection of 256 directories,
// or a collection of 256x256 directories.

// There are 3 options:
// 
// Level 0: All cache files are in a single directory (the cache root).
// Level 1: Cache files are located in 256 sub-directories numbered 0 .. 255 directly under the cache root.
// Level 2: There are two levels of sub-directories under the cache root. 
//          The 1st level has 256 sub-directories numbered 0 .. 255 directly under the cache root. 
//          The 2nd level allows for up to 256 sub-directories (0..255) under each level-1 directory. 
//          Cache files are stored in the leaf directories. 

// For this example, given that we anticipate a larger number of hash entries, we choose a "level 2" cache.
cache := CkCache_Create();

// We can also spread the cache among several root directories, but for this example we'll only use one root directory.
// Call AddRoot once for each root directory.
CkCache_AddRoot(cache,'c:/temp/myCache');
CkCache_putLevel(cache,2);

// Add some key/values to the persisted hash table (i.e. the cache).
// The eTag is optional metadata.
key := 'apple';
eTag := '';
itemValue := 'macos';
success := CkCache_SaveTextNoExpire(cache,key,eTag,itemValue);

// Add more items..
success := CkCache_SaveTextNoExpire(cache,'microsoft','','windows');
success := CkCache_SaveTextNoExpire(cache,'google','','android');

// Lookup items:
key := 'microsoft';
itemValue := CkCache__fetchText(cache,key);
Memo1.Lines.Add(key + ': ' + itemValue);

key := 'apple';
itemValue := CkCache__fetchText(cache,key);
Memo1.Lines.Add(key + ': ' + itemValue);

CkCache_Dispose(cache);

end;